1. Field of the Invention
The present invention relates to bus architectures, and more particularly to a bus bridge architecture for interconnecting a plurality of devices through bridge logic to a PCI bus.
2. Description of the Related Art
Personal computers are increasingly being used for multimedia and communication applications. Examples of multimedia applications include applications with audio and/or video presentation components. Examples of communication applications include e-mail clients, web browsing, and intranet access, among others.
A multimedia application which includes audio typically interfaces to an audio source and/or destination device. Audio sources which may need to be supported include television, radio, microphones, and compact disks, as well as telephony and modem applications. however, multimedia computers must do more than merely function as an audio switching unit from source to speakers and headphones. Multimedia computers must also provide the user the capability to manipulate the audio signals in an interactive fashion. Thus, it is desirable to provide a personal computer which can support music synthesizers, audio mixers, sound effects synthesizers, and the like, in addition to any associated stereo and mono codes. Such functionality may be provided on a centralized audio chipset, which may receive signals from all audio sources.
It is frequently desirable to provide multiple audio and/or communication devices, and particularly telephony and modem devices, on an expansion card. However, most modem multimedia computers employ the Peripheral Component Interconnect (PCI) bus architecture. While the PCI bus accommodates expansion slots for add-in cards, each card is permitted to provide only one load on the bus. Accordingly, if multiple devices, such as multiple modems, are desired to be supported on a given expansion board, the devices must connect to a bus on-board the bridge device. A bridge device is required to interface between the devices connected to the on-board bus and the computer's expansion bus.
A variety of options is available for an on-board bus connecting to a computer expansion bus. A standard expansion bus, such as an Industry Standard Architecture (ISA), Extended Industry Standard Architecture (EISA) or Microchannel Architecture (MCA) may be employed. Bus bridges connecting each of these buses to a PCI bus are known. However, while such bus interconnections are an option, each of the above mentioned buses is an asynchronous bus requiring a bus master and relatively complex arbitration mechanism to support multiple devices. In addition, each of these bus standards is relatively slow. In time-critical isochronous functions, such as multimedia audio, this can introduce undesirable delays, both when the devices must communicate with one another and when they must communicate across the bus bridge on the PCI bus to the host computer. However, there is a large base of, for example, ISA bus compatible multimedia devices which could frustrate any move to a new standard. In addition, devices according to each of the above standards require a large number of pins to interface with the PCI bus.
Accordingly, there is a need for a more efficient bus bridge interconnect architecture for multimedia-related devices for interconnection and for interfacing to a standard bus, which also maintains support for an existing body of devices compatible with a previous standard and minimizes the number of pins required for connection. More particularly, there is a need for a bus mechanism for functionally coupling a plurality of multimedia devices to a PCI bus, while providing support for devices employing the ISA standard.